The present invention relates to the selective storage and release of oxygen and gas separation by ceramic materials. In particular, the present invention relates to methods of elevated temperature air separation, oxygen storage, or any process related to temperature or oxygen partial-pressure dependent absorption and desorption of oxygen with ceramic materials.
Recently ceramic materials have been increasingly researched due to their reversible oxygen storage/release capacities (OSC) at elevated-temperatures. New ceramic materials for elevated-temperature air separation are strong candidates to compete with cryogenic distillation for commercial air separation and are also being researched for components to improve automotive exhaust catalysts, solar water splitting, hydrogen-oxygen fuel cells, various non-aerobic oxidation processes, and assorted high-temperature production processes that require high-purity oxygen (e.g. steel, copper, plastics, glass, etc.). Elevated temperature air separation methods have been projected to have 20-30% less capital and operation cost, while being significantly more energy efficient, than conventional air separation methods. The development of improved oxygen storage or carrier materials is also critical to the success of new energy related technologies such as “oxy-fuel” and “chemical looping” combustion systems for “clean coal” energy production, automotive pollution reduction, hydrogen-oxygen fuel cells, solar water splitting, and to improve the efficiency and cost of various production processes (e.g. steel, copper, plastics), and the production of synthesis gas (H2, CO) by partial oxidation of methane.
Ideal materials have large values of OSC (typically measured in moles of oxygen per weight of material) and their absorption/desorption of oxygen occurs over a narrow temperature range at near atmospheric conditions. Additional properties, such as oxygen partial pressure dependence of absorption/desorption, exothermic absorption and endothermic reduction, stability/recoverability in strong reducing conditions (e.g. CO and H2 atmospheres at high-temperatures), are also desired and being researched for various applications. Commercially, fluorite Ce1−xZrxO2 compositions have been the recent ceramic OSC materials of choice for air separation, which function around 500° C. and have OSCs of ˜400-500 μmol-O/g in oxygen atmospheres or as high as 1500 μmol-O/g with 20% H2 reversible reduction. Recent studies with Ce1−xCrxO2 have further boosted the OSC of the fluorite structure to as high as 2500 μmol-0/g in air and hydrogen atmospheres but require considerably higher reduction temperatures (550-700° C.) and contain poisonous Cr6+. Currently, RBaCo4O7+δ (R=Y, Dy, Ho, Er, Tm, Yb, and Lu) and YBaCo4−xAlxO7+δ have the best reported OSC at low-temperature, which have storage up to ˜2700 μmol-O/g and completely desorb at ˜400-425° C. in O2. The ease of reversible phase transitions between the hexagonal P63mc YBaCo4O7 and orthorhombic Pbc21 YBaCo4O8.1 phases (which is a mixture of tetrahedrally and octahedrally coordinated cobalt) is responsible for its oxygen storage behavior.
RMnO3 (R=rare earths) and their competing hexagonal and perovskite crystal structures have been studied for over fifty years. Conventionally, the formation of the perovskite phase versus the hexagonal phase is governed primarily by the size of the rare-earth ion in RMnO3 (with constant Mn3+ size). During high-temperature solid state synthesis in air, the perovskite phase forms easily with larger rare-earth elements (e.g. La, Pr, Nd, Sm, Gd, Tb, and Dy), while smaller size rare-earths (e.g. Ho, Er, Tm, Yb, Lu, and Y) favor the hexagonal phase. It has been observed that the perovskite phase is stable for a tolerance factor,
      t    =                  (                  R          -          O                )                              2                ⁢                  (                      Mn            -            O                    )                      ,in the range of 0.855≦t≦1 (calculated at room temperature using Shannon's ionic size values), where the perovskite structure is increasingly distorted as it approaches this lower limit and results in the transition to the hexagonal phase at t<0.855. Recently, Zhou et al. suggested that the relative large difference in density between the perovskite and hexagonal phases can have a large impact on the formation of the perovskite versus the hexagonal near the lower limit of the tolerance factor. Regardless, DyMnO3 and YMnO3 have tolerance factors of 0.857 and 0.854, respectively, and will tend to form the perovskite and hexagonal phases, respectively, under normal solid state reaction synthesis. Thus the average (R—O) bond length of substituted samples causes Dy1−xYxMnO3 to be on the cusp of this phase transition and, as further discussed herein, results in a mixed state under synthesis in air.
U.S. Patent Application Publication No. 2009/0206297 to Karppinen, et al. discloses an oxygen excess type metal oxide expressed with the following formula (1) and exhibiting high speed reversible oxygen diffusibility whereby a large amount of excess oxygen is diffused at a high speed and reversibly in a low temperature region:AjBkCmDnO7+δ  (1)
where
A: one or more trivalent rare earth ions and Ca
B: one or more alkaline earth metals
C, D: one or more oxygen tetra-coordinated cations among which at least one is a transition metal, where j>0, k>0, and, independently, m≧0, n≧0, and j+k+m+n=6, and 0<δ≦1.5. The metal oxide has high oxygen diffusibility and large oxygen non-stoichiometry at a low temperature region (500° C. or less, in particular 400° C. or less) and a ceramic is disclosed for oxygen storage and/or an oxygen selective membrane comprised of the metal oxide. The Karppinen, et al. metal oxide has a high 2:1 ratio of expensive and poisonous Co (where C=Co) to the less expensive trivalent rare earth ions and alkaline earth metals. In addition, the compounds disclosed contain B=Ba that is highly reactive with CO2 and water vapor present in air and easily decompose when heated just above their optimal OSC temperature. The Karppinen, et al. metal oxide has the disadvantages of being expensive and not thermodynamically stable or safe.
One disadvantage of currently used materials for oxygen storage or air separation is that they depend on the creation of oxygen ion vacancies or interstitial sites at high-temperatures in order to store the oxygen. Currently, the majority of materials for air separation use high-pressure (zeolites) or low-temperature (cryogenic distillation) methods consuming large amounts of energy. However, roughly over 80% of commercially produced oxygen is used in high-temperature industrial productions process and many developing applications of OSC materials operate at high-temperatures as well. For any of these current and potential systems, the redirection of the large amounts of waste heat generated from all these methods to ceramic OSC materials for onsite air separation, would undoubtedly have potential net energy, economic, and waste advantages versus conventional methods. The majority of new ceramic OSC materials for such applications rely on the creation of oxygen ion vacancies or interstitial sites at high-temperatures; however, this is a poor mechanism for currently known materials due to the high-temperatures (up to 1000° C.) and large temperature gradients (˜300-800° C.) required for moderate oxygen storage capacities (less than 500 μmol-O/g).
Therefore, there remains a need for a method for the selective storage and release of oxygen that does not require extreme temperatures or large temperature gradients so as to reduce cost and energy consumption, as well as a method that is able to increase the amount of oxygen that is able to be stored.